Support `pod.spec.containers.securityContext` specification

[Kohei Watanabe]

We want to specify pod.spec.containers.securityContext so I'm glad if this is supported:

e.g. Superset supports that:
https://github.com/apache/superset/blob/8c32c6da169afec312923e516850d90a69e78f46/helm/superset/values.yaml#L336